Transient response of the flexible blade of horizontal-axis wind turbines in wind gusts and rapid yaw changes

Abstract

In this paper, the aeroelastic analysis of a large scale wind turbine rotor is performed with the aim of studying transient performance of turbine in extreme wind conditions, such as wind gusts and rapid yaw changes. The effect of the presence and/or lack of blade pitch control system on output power, rotor thrust, and blade deformation in sudden change of wind speed are investigated. The NREL 5 MW offshore wind turbine is used as the baseline case. In this regard, the modal approach is implemented for modeling the flexible blade structure with tension, bending and torsion degrees of freedom. The unsteady vortex lattice method is employed to obtain the aerodynamic loads. Moreover, the Lagrange equation is utilized to derive the governing dynamic equations and the yielded nonlinear system of equations is solved in time domain. Structure and aerodynamic models are validated by using standard cases. Wind turbine response to different scenarios for sudden change of wind speed and direction are investigated. The results show that the sudden change in wind speed leads to the emergence of overshoots (or undershoots) in output power as high as 40% (and 20%) for active pitch control case and nearly 25% and 20% gradual increment of thrust and tip deformations for inactive case. Time delay of recovering to the new steady-state output power is about 4 s (corresponding to one rotor revolution) and independent from the state of pitch control system. With sudden change of yaw angle, the turbine transient response gradually attains a new steady-state after a time delay of about 5 s. Moreover, results indicate that the output power and rotor thrust have a cyclical variation with the frequency of rotor rotation due to the asymmetric wake configuration.